Dipole Interaction in Triplet Superconductivity with Horizontal Line Nodes; Orientation of the Superconducting Order Parameter in Sr2RuO4

Dipole interaction in the triplet superconductivity is studied. If the Cooper pairs are formed by the electrons at $\mib{r}$ and $\mib{r}+(\pm\frac{a}{2},\pm\frac{a}{2},\pm\frac{c}{2})$ in the body centered tetragonal lattice, (in this case the line nodes run horizontally on the cylindrical Fermi surface), the dipole energy is low when the ${\mib{d}}$-vector is perpendicular to the direction of the angular momentum of the Cooper pairs ($\mib{l}$-vector). This result is in contrast with the dipole energies in the ABM state of superfluid $^3$He, where $\mib{d}$-vector is forced to be parallel or antiparallel to the $\mib{l}$-vector by the dipole interaction. The recent NQR experiment in Sr$_2$RuO$_4$ by Ishida et al. can be explained by this result.